Enhanced mid-infrared light-matter coupling in heavily-doped p-germanium due to interplay between plasmonic and inter-subband contributions

The development of germanium-based mid-infrared (MIR) optoelectronic technologies is central to advancing compact, high-performance on-chip systems. While plasmonic field enhancement in heavily-doped semiconductors is widely exploited to intensify light-matter interactions, yet p- type germanium has been discarded due to its complex valence band structure. Here, we show that heavily-doped p- type germanium exhibits a dual light-matter interaction, where an ε-near-zero band delineates a plasmonic-like regime below the plasma frequency and a broadband absorption (dominated by inter-subband transitions) above it. This duality extends the spectral range of strong interaction beyond conventional Drude plasmonics. Using the Czochralski method, we have grown high-quality heavily-doped germanium single crystals with gallium concentrations up to ~2×10 20 cm −3 , exhibiting a high, near 100% acceptor activation. Mid-infrared spectroscopic ellipsometry and reflectance measurements reveal a continuous blue shift of the plasma wavelength with increasing dopant concentration, up to 4.5 μm, the shortest wavelength reported in Ge to date. Our findings manifest the potential of heavily-doped p- type bulk germanium crystals for applications that benefit from strong mid-infrared absorption and high crystal lattice quality.